Phosphatidylinositol (hereinafter abbreviated as “PI”) is one of a number of phospholipids found in cell membranes. In recent years it has become clear that PI plays an important role in intracellular signal transduction. Cell signaling via 3′-phosphorylated phosphoinositides has been implicated in a variety of cellular processes, e.g., malignant transformation, growth factor signaling, inflammation, and immunity (Rameh et al (1999) J. Biol Chem, 274:8347-8350). The enzyme responsible for generating these phosphorylated signaling products, phosphatidylinositol 3-kinase (also referred to as PI 3-kinase or PI3K), was originally identified as an activity associated with viral oncoproteins and growth factor receptor tyrosine kinases that phosphorylate phosphatidylinositol (PI) and its phosphorylated derivatives at the 3′-hydroxyl of the inositol ring (Panayotou et al (1992) Trends Cell Biol 2:358-60).
Phosphoinositide 3-kinases (PI3K) are lipid kinases that phosphorylate lipids at the 3-hydroxyl residue of an inositol ring (Whitman et al (1988) Nature, 332:664). The 3-phosphorylated phospholipids (PIP3s) generated by PI3-kinases act as second messengers recruiting kinases with lipid binding domains (including plekstrin homology (PH) regions), such as Akt and phosphoinositide-dependent kinase-1 (PDK1). Binding of Akt to membrane PIP3s causes the translocation of Akt to the plasma membrane, bringing Akt into contact with PDK1, which is responsible for activating Akt. The tumor-suppressor phosphatase, PTEN, dephosphorylates PIP3 and therefore acts as a negative regulator of Akt activation. The PI3-kinases Akt and PDK1 are important in the regulation of many cellular processes including cell cycle regulation, proliferation, survival, apoptosis and motility and are significant components of the molecular mechanisms of diseases such as cancer, diabetes and immune inflammation (Vivanco et al (2002) Nature Rev. Cancer 2:489; Phillips et al (1998) Cancer 83:41).
The PI3 kinase family comprises at least 15 different enzymes sub-classified by structural homology and are divided into 3 classes based on sequence homology and the product formed by enzyme catalysis. The class I PI3 kinases are composed of 2 subunits: a 110 kd catalytic subunit and an 85 kd regulatory subunit. The regulatory subunits contain SH2 domains and bind to tyrosine residues phosphorylated by growth factor receptors with a tyrosine kinase activity or oncogene products, thereby inducing the PI3K activity of the p110 catalytic subunit which phosphorylates its lipid substrate. Class I PI3 kinases are involved in important signal transduction events downstream of cytokines, integrins, growth factors and immunoreceptors, which suggests that control of this pathway may lead to important therapeutic effects such as modulating cell proliferation and carcinogenesis. Class I PI3Ks can phosphorylate phosphatidylinositol (PI), phosphatidylinositol-4-phosphate, and phosphatidylinositol-4,5-biphosphate (PIP2) to produce phosphatidylinositol-3-phosphate (PIP), phosphatidylinositol-3,4-biphosphate, and phosphatidylinositol-3,4,5-triphosphate, respectively. Class II PI3Ks phosphorylate PI and phosphatidylinositol-4-phosphate. Class III PI3Ks can only phosphorylate PI.
The main PI3-kinase isoform in cancer is the Class I PI3-kinase, p110α. (U.S. Pat. Nos. 5,824,492; 5,846,824; 6,274,327). Other isoforms are implicated in cardiovascular and immune-inflammatory disease (Workman P (2004) “Inhibiting the phosphoinositide 3-kinase pathway for cancer treatment” Biochem Soc Trans 32:393-396; Patel et al (2004) “Identification of potent selective inhibitors of PI3K as candidate anticancer drugs” Proceedings of the American Association of Cancer Research (Abstract LB-247) 95th Annual Meeting, March 27-31, Orlando, Fla., USA; Ahmadi K and Waterfield Md. (2004) “Phosphoinositide 3-Kinase: Function and Mechanisms” Encyclopedia of Biological Chemistry (Lennarz W J, Lane M D eds) Elsevier/Academic Press).
Several components of the PI3-kinase/Akt/PTEN pathway are implicated in oncogenesis. In addition to growth factor receptor tyrosine kinases, integrin-dependent cell adhesion and G-protein coupled receptors activate PI3-kinase both directly and indirectly through adaptor molecules. Functional loss of PTEN (the most commonly mutated tumor-suppressor gene in cancer after p53), oncogene mutations in PI3 kinase (Samuels et al (2004) Science 304:554), amplification of PI3-kinase and overexpression of Akt have been established in many malignancies. In addition, persistent signaling through the PI3-kinase/Akt pathway by stimulation of the insulin-like growth factor receptor is a mechanism of resistance to epidermal growth factor receptor inhibitors such as AG1478 and trastuzumab. Oncogenic mutations of p110alpha have been found at a significant frequency in colon, breast, brain, liver, ovarian, gastric, lung, and head and neck solid tumors. PTEN abnormalities are found in glioblastoma, melanoma, prostate, endometrial, ovarian, breast, lung, head and neck, hepatocellular, and thyroid cancers.
The levels of phosphatidylinositol-3,4,5-triphosphate (PIP3), the primary product of PI3-kinase activation, increase upon treatment of cells with a variety of agonists. PI3-kinase activation, therefore, is believed to be involved in a range of cellular responses including cell growth, differentiation, and apoptosis (Parker et al (1995) Current Biology, 5:577-99; Yao et al (1995) Science, 267:2003-05). Though the downstream targets of phosphorylated lipids generated following PI3 kinase activation have not been well characterized, emerging evidence suggests that pleckstrin-homology domain- and FYVE-finger domain-containing proteins are activated when binding to various phosphatidylinositol lipids (Sternmark et al (1999) J Cell Sci, 112:4175-83; Lemmon et al (1997) Trends Cell Biol, 7:237-42). In vitro, some isoforms of protein kinase C (PKC) are directly activated by PIP3, and the PKC-related protein kinase, PKB, has been shown to be activated by PI3 kinase (Burgering et al (1995) Nature, 376:599-602).
The initial purification and molecular cloning of PI3 kinase revealed that it was a heterodimer consisting of p85 and p110 subunits (Otsu et al (1991) Cell 65:91-104; Hiles et al (1992) Cell 70:419-29). Since then, four distinct Class I PI3Ks have been identified, designated PI3K α (alpha), β (beta), δ (delta), and ω (gamma), each consisting of a distinct 110 kDa catalytic subunit and a regulatory subunit. More specifically, three of the catalytic subunits, i.e., p110.alpha., p110.beta. and p110.delta., each interact with the same regulatory subunit, p85; whereas p110.gamma. interacts with a distinct regulatory subunit, p101. As described below, the patterns of expression of each of these PI3Ks in human cells and tissues are also distinct.
The cellular functions of the individual isoforms of PI3 kinases are not completely elucidated. Bovine p110 alpha was described after cloning as related to the Saccharomyces cerevisiae protein: Vps34p, a protein involved in vacuolar protein processing. The recombinant p110 alpha product was also shown to associate with p85 alpha, to yield a PI3K activity in transfected COS-1 cells (Hiles et al. (1992), Cell, 70:419-29). A second human p110 isoform was cloned and designated p110 beta (Hu et al (1993) Mol Cell Biol 13:7677-88). This isoform is said to associate with p85 in cells, and to be ubiquitously expressed, as p110 beta mRNA has been found in numerous human and mouse tissues as well as in human umbilical vein endothelial cells, Jurkat human leukemic T cells, 293 human embryonic kidney cells, mouse 3T3 fibroblasts, HeLa cells, and NBT2 rat bladder carcinoma cells. Identification of the p110 delta isoform of PI3 kinase is described in Chantry et al., J Biol Chem, 272:19236-41 (1997). It was observed that the human p110 delta isoform is expressed in a tissue-restricted fashion. It is expressed at high levels in lymphocytes and lymphoid tissues, suggesting that the protein might play a role in PI3 kinase-mediated signaling in the immune system (U.S. Pat. Nos. 5,858,753; 5,822,910;5,985,589; WO 97/46688; and Vanhaesebroeck et al (1997) Proc Natl Acad Sci USA, 94:4330-5).
In each of the PI3K alpha, beta, and delta subtypes, the p85 subunit acts to localize PI3 kinase to the plasma membrane by the interaction of its SH2 domain with phosphorylated tyrosine residues (present in an appropriate sequence context) in target proteins (Rameh et al (1995) Cell, 83:821-30). Two isoforms of p85 have been identified, p85alpha, which is ubiquitously expressed, and p85 beta, which is primarily found in the brain and lymphoid tissues (Volinia et al (1992) Oncogene, 7:789-93). Association of the p85 subunit to the PI3 kinase p110 alpha, beta, or delta catalytic subunits appears to be required for the catalytic activity and stability of these enzymes. In addition, the binding of Ras proteins also upregulates PI3 kinase activity. Cloning of p110 gamma revealed further complexity within the PI3K family of enzymes (Stoyanov et al (1995) Science, 269:690-93). The p110 gamma isoform is closely related to p110 alpha and p110 beta (45-48% identity in the catalytic domain), but does not make use of p85 as a targeting subunit. Instead, PI3K contains an additional domain termed a “pleckstrin homology domain” near its amino terminus. This domain allows interaction of p110 gamma with the beta, gamma subunits of heterotrimeric G proteins and this interaction appears to regulate its activity. The p101 regulatory subunit for PI3 Kgamma was originally cloned in swine, and the human ortholog identified subsequently (Krugmann et al (1999) J Biol Chem, 274:17152-8).
Thus, PI3 kinases can be defined by their amino acid identity or by their activity. Additional members of this growing gene family include more distantly related lipid and protein kinases including Vps34 TOR1, and TOR2 of Saccharomyces cerevisiae (and their mammalian homologs such as FRAP and mTOR), the ataxia telangiectasia gene product (ATR) and the catalytic subunit of DNA-dependent protein kinase (DNA-PK). See generally, Hunter (1995) Cell, 83:1-4.
PI3 kinase also appears involved in leukocyte activation. A p85-associated PI3 kinase activity has been shown to physically associate with the cytoplasmic domain of CD28, which is an important costimulatory molecule for the activation of T-cells in response to antigen (Pages et al (1994) Nature, 369:327-29; Rudd, (1996) Immunity 4:527-34). Activation of T cells through CD28 lowers the threshold for activation by antigen and increases the magnitude and duration of the proliferative response. These effects are linked to increases in the transcription of a number of genes including interleukin-2 (IL2), an important T cell growth factor (Fraser et al (1991) Science, 251:313-16). Mutation of CD28 such that it can no longer interact with PI3 kinase leads to a failure to initiate IL2 production, suggesting a critical role for PI3 kinase in T cell activation.
Inhibition of class I PI3 kinase induces apoptosis, blocks tumor induced angiogenesis in vivo, and increases the radiosensitivity of certain tumors. At least two compounds, LY294002 and wortmannin, have been widely used as PI3 kinase inhibitors. These compounds, however, are nonspecific PI3K inhibitors, as they do not distinguish among the four members of Class I PI3 kinases. For example, the IC50 values of wortmannin (U.S. Pat. No. 6,703,414) against each of the various Class I PI3 kinases are in the range of 1-10 nanomolar (nM). LY294002 (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) is a well known specific inhibitor of class I PI3 kinases and has anti-cancer properties (Chiosis et al (2001) Bioorganic & Med. Chem. Lett. 11:909-913; Vlahos et al (1994) J. Biol. Chem. 269(7):5241-5248; Walker et al (2000) Mol. Cell. 6:909-919; Fruman et al (1998) Ann Rev Biochem, 67:481-507). However, the anti-cancer applications of LY294002 are severely limited by its lack of aqueous solubility and its poor pharmacokinetics. Moreover, LY294002 has no tissue specific properties and has been demonstrated to be rapidly metabolized in animals. Because of these factors, LY294002 would need to be administered at frequent intervals and thus has the potential to also inhibit PI3 kinases in normal cells thereby leading to undesirable side effects.
There continues to be a need for class I PI3 kinase inhibitors with improved pharmacokinetic and pharmacodynamic properties. The PI3 kinase/Akt/PTEN pathway is thus an attractive target for cancer drug development since such agents would be expected to inhibit proliferation, reverse the repression of apoptosis and surmount resistance to cytotoxic agents in cancer cells. PI3 kinase inhibitors have been reported (Yaguchi et al (2006) Jour. of the Nat. Cancer Inst. 98(8):545-556; U.S. Pat. Nos. 6,608,056; 6,608,053; 6,838,457; 6,770,641; 6,653,320; 6,403,588; WO 2004017950; US 2004092561; WO 2004007491; WO 2004006916; WO 2003037886; US 2003149074; WO 2003035618; WO 2003034997; US 2003158212; EP 1417976; US 2004053946; JP 2001247477; JP 08175990; JP 08176070). Wortmannin analogs have PI3 kinase activity in mammals (U.S. Pat. No. 6,703,414; WO 97/15658).